Currently, three genera and four species are known in horseshoe crabs. Of these horseshoe crabs, lectin, an erythrocyte aggregation factor, is isolated and purified from three species, an American horseshoe crab (Limulus polyphemus), a Japanese horseshoe crab (Tachypleus tridentatus), and an Indian horseshoe crab (Carcinoscrpius rotunda cauda). The lectins obtained from these horseshoe crabs have poor specificity to sialic acids. In addition, it is reported that the lectins also recognize other sugars [Marchalonis, J. J. & Edelman, G. M., 1968: J. Mol. Biol. 32: 453-465; Annie-Claude Roche and Michel Monsigny, 1974: Biochemica et Biophysica Acta. 371: 242-254; T. P. Nowak and S. H. Barondes, 1975: Biochemica et Biophysica Acta. 393: 115-123; Shimizu, S., Ito, M., & Niwa, M., 1977, Biochem. Biophys. Acta 393: 115-123; Shimizu, S., Ito, M., Takahashi, N., & Niwa, M., 1979," Biomedical Applications of the Horseshoe Crab (Limulidae) pp. 625-639, Cohen, E., ed., Alan R. Liss, Inc., New York; F. Shishikura and K. Sekiguchi., 1983, 93:1539-1546; S. Bishayee and D. Thambi Dorai., 1983, Biochemica et Biophysica Acta. 623: 89-97].